Process for preparing permanent magnets by division of crystals

ABSTRACT

Method for the preparation of permanent magnets at room temperature from an alloy containing at least a mixture of iron (Fe), boron (B) and rare earths (RE) including Yttrium, and for which there is a temperature range wherein said alloy is in two phases; one solid and brittle and the other one liquid. The method comprises heating said alloy under controlled atmosphere at a temperature sufficient to reach said temperature range, treating said alloy, and finally, optionally, allowing the treated alloy to cool. The method being characterized on the one hand in that said Fe-B-Re alloy is in a massive form, and on the other hand, in that the treatment of said massive alloy is carried out by welding of the magnetic solid phase Fe-B-Re.

The invention relates to a new process for making high-performancepermanent magnets by division of the crystals of a magnetic phase in analloy

In the manufacture of permanent magnets, it was well known to employmetal alloys of iron (Fe)-Boron (B) also including Rare Earths (RE). Atthe present time, there are essentially two types of preocess formanufacturing such magnets.

In the first process employing powder metallurgy, described in EuropeanPatent Applications EP-A-0 101 552, 0 106 948 and 0 126 802, aniron-boron-rare earth alloy is made which is ground in the form ofpowder, then oriented in a magnetic field which is compressed cold,which is sintered and finally which is subjected to a heat treatment.Although the magnets obtained in this way present excellent properties,this process nonetheless presents noteworthy drawbacks. In fact, theslightest pollution considerably alters the final properties. Now,pollution of the powder by the atmosphere is extremely rapid; thistherefore necessitates working under a controlled atmosphere at ambienttemperature, which increases manufacturing costs. In addition, it isnecessary to employ a grinding phase. Now, the powders used present ahigh reactivity, particularly with respect to air, which unfortunatelyinvolves considerable risks of explosion and of fire.

The second process employs the technique of micro-crystallization. Thistechnique, described in European Patents EP-A-0 125 752 or EP-A-0 133758, essentially consists in melting an alloy of the type in question,then in subjecting it to a treatment of rapid hardening on roller, incrushing and hot-pressing, or in coating the material obtained in aresin. This technique of very fine jet of liquid at high temperaturehardened on cold roller unfortunately leads to isotropic magnets, unlessthey are subjected to an operation of creep and of recrystallizationwhich is always difficult to carry out in a continuous process. Inaddition, as a high-temperature fusion is employed with ejection of avery fine liquid, an appropriate apparatus must be used and operationmust be carried out in a controlled atmosphere in large-dimensionedenclosures with all the drawbacks that this comprises.

Finally, in these two techniques, one necessarily passes through a phasein the course of which the alloy is considerably divided.

The invention overcomes these drawbacks. It envisages a process of thetype in question which is easy to carry out, employs conversions of moreeconomical raw materials, and leads to materials having improvedproperties.

This process for preparing permanent magnets at ambient temperature froman alloy containing at least one mixture of Iron (Fe), Boron (B) andanother element selected from the group that includes rare earth (RE)and yttrium (Y), and for which there is a temperature range inside whichsaid alloy is in two phases: one solid and fragile, and the otherliquid. In this process:

said alloy is heated in a controlled atmosphere at a sufficienttemperature to attain the said temperature range;

then this alloy is treated;

and finally, the treated alloy is possibly left to cool.

The process is characterized:

on the other hand, in that said Fe/B/RE alloy is in bulk-state form;

and, on the other hand, in that treatment of this massive alloy iseffected by welding the magnetic Fe/B/RE solid phase.

In other words, the invention consists firstly in no longer employing analloy in the form of powder but a bulk alloy comprising two phases, thenin heating this bulk alloy, and finally in subjecting it to highmechanical stresses to induce a welding at a temperature allowing thefracture of the magnetic crystals into particles dimensioned on theorder of tens of microns and finally, favorably, in cooling this alloy.

In the following specification and claims, the term "controlledatmosphere" is used to designate an atmosphere of which the compositionis monitored; in practice, it is question of an atmosphere of noblegases or vacuum, and that in order to avoid reactgions with the RareEarths;

The term "welding" designates a mechanical treatment applied to thebinary-phase (part liquid/part solid) metallic alloy, intended toprovoke grain refining of this alloy; treatments of forging, hammering,rolling, extrusion, vibroramming (ramming by vibrations), may bementioned.

Advantageously, in practice:

the bulk-state alloy is a ternary alloy based on Iron, Boron and RareEarths, the group of rare earths in this case also including yttrium;

in practice, particularly for substantial reasons of economy and ofmechanical properties, the Rare Earth is selected from the groupconstituted by Neodymium and Praseodymium, which in that case is in alarger proportion;

the respective proportions of the different constituents of this alloy,which may also contain other agents for forming eutectics, such asAluminium or Gallium, correspond to the usual proportions, particularlythose described in the European Patent Applications mentioned in thepreamble;

the alloy is in the form of bulk-state ingots, possibly in the form ofmassive pieces; in that way, in other words, during application of themechanical stresses of welding, the magnetic crystals are broken hot inthe liquid which surrounds them in final phase;

heating of the massive alloy can be effected by any known means, such asJoule effect or induction, the alloy being able to be either in a rightenvelope or in vacuo or in a noble gas;

the bulk alloy thus heated is welded either in vacuo or in a noble gas,or in a non-reactive liquid, or even in a tight envelope that mayundergo the mechanical and thermal treatments, such as for example andenvelope of mild Iron or an alloy based on Iron;

heating is effected at a temperature of between 400° and 1050° C.,preferably in the vicinity of 700° C., in any case at a sufficienttemperature to attain the plasticity of the non-magnetic eutectic phase;it has been observed that, if the temperature is lower than 400° C., thealloy is reduced to powder, this returning to the first technique setforth in the preamble, whilst, if this temperature exceeds 1050° C., thephenomenon of welding is no longer obtained, as the magnetic grainsbecome too malleable and enlarge as the treatment continues;

the mechanical stresses of welding are developed as already stated, byforging, hammering, extrusion, rolling or any other thermo-mechanicaltreatment; it has been observed that the size of the magnetic crystalsobtained results from the rate of welding applied in the products; ithas thus been observed that good results are obtained with a deformationratio higher than ten, advantageously of the order of twenty five;

after possible cooling, the treated alloy undergoes a treatment ofannealing and/or of tempering at temperatures of between 600° and 1000°C. and even more, preferably between 700° and 900° C., which improvesand stabilizes the magnetic properties, particularly the coercivity.

In other words, the fundamental characteristic of the invention consistsin not employing an alloy in the form of powder but a bulk alloy, whichis much more economical and less dangerous, then in treating this bulkalloy by welding, which no longer necessitates employing complex andexpensive apparatus.

The manner in which the invention may be carried out and the advantagesfollowing therefrom will be more readily seen from the followingembodiments given by way of non-limiting indication in support of theaccompanying single Figure.

BRIEF DESCRIPTION OF THE DRAWING

The sole drawing Figure schematically shows an installation for carryingout the process according to the invention.

This installation basically comprises an anvil 1 on which rests aholding ring 2 surrounded by a glass enclosure 3, defining a tightchamber 4, connected by the inlet 5 to a source of Argon (not shown).The top of the tight chamber comprises an opening 6 through which thehammer 7 of the outside striking assembly 8 may pass through an O-ring9. The sample 10 rests on the anvil 1 around the ring 2 in which thehammer slides. The glass enclosure 3 is surrounded by turns 11 forheating by induction.

EXAMPLE 1

In known manner, a massive sample (washer, moulded cylinder, case ingot,shot, . . . ) is prepared from an Iron/Boron/Rare Earth alloy,essentially comprising for one hundred atoms:

78 atoms of Iron;

6 atoms of Boron;

15.5 atoms of Neodymium;

0.5 atom of Aluminium.

Pieces of alloy of any shape are placed on the anvil 1, within the ring2. Argon is injected at 5 and by induction (11), the plate 10 is heatedto 650° C. for five minutes. When this temperature is attained, theplate 10 is hammered for two minutes by the assembly 7, 8, developing apower of six Joules per strike at a rate of one thousand eight hundredstrikes per minute. A bulk-state plate of twenty millimetres diameterand five millimetres thickness is obtained.

It should be noted that, at that temperature, the fusible phase is apoorly identified mixture of metallic phases and even possibly of salts(fluorides and chlorides of Rare Earths) and of oxides. The principalmagnetic phase tetragonal Nd₂ Fe₁₄ B remains present up to at least1050° C. and during all the mechanical treatments or annealing.

It is then left to cool for three minutes down to 70° C.

The plate thus obtained presents an intrinsic coercitive field of 300kiloAmperes per metre (300kA/m), a density equal to 7.6 and a remanentinduction of 0.55 Tesla.

The material obtained presents a quadratic, i.e. tetragonal crystallinestructure Nd₂ Fe₁₄ B.

EXAMPLE 2

The same sample as in Example 1 is subjected to an additional operationof annealing for about thirty minutes at 800° C. carried out in chamber4.

A magnet having an intrinsic coercitive field of 1000 kA/m, a remanentinduction of 0.85 Tesla, an internal energy of 1000 kiloJoules per cubitmetre and a density of 7.6, is thus obtained.

EXAMPLE 3

Example 2 is repeated, applying during the annealing treatment aconstant, unidirectional pressure on the sample 10. Strongly anisotropicmagnets are thus obtained.

In these three Examples 1 to 3, the hammering operation is undertakenonly when the ancillary phases are sufficiently plastic in order toinduce only refining of the crystals responsible for the magneticproperties.

EXAMPLES 4

Three kilos of a bulk NdFeB alloy, of atomic composition: Nd₁₅.5 Fe₇₈ B₆Al₀.5, are made. This bulk alloy is cast into a mild Iron recipienthaving a diameter of sixty millimetres, a length of two hundredmillimetres and a thickness of six millimetres.

After coolilng, the recipient is hermetically closed.

After heating the massive alloy in its container to 750° C., the wholeis extruded in an extruder of appropriate shape, for example in flatform. A rectangular bar of twenty five by seven millimetres and severalmetres long is then obtained, with a deformation ratio of 25 and anapplied pressure of 13 kBar.

The magnet obtained is then cut to the desired length.

This magnet presents the following characteristics:

coercitive field H_(Ci) : 700 kA/m,

coercitive induction field H_(CB) : 400 kA/m,

remanent induction Br : 0.75 Tesla

internal engergy BH_(max) : 100 kJ/m³

these measurements being made in directions perpendicular to thedirection of extrusion.

An operation of annealing is then carried out in a controlled atmosphereof rare gas.

The following characteristics are then obtained:

H_(Ci) : 1000 kA/m

H_(CB) : 480 kA/m

Br: 0.85 Tesla

BH: 120 kJ/m³

In brief, it has been observed that the refining of the crystals of thealloy notably increases the coercivity of the whole. Moreover, asindustry most often demands anisotropic permanent magnets, anisotropy isobtained as has already been stated by the application of a strongunidirectional pressure on the material treated, the eutectic phasebeing in plastic phase.

It has been observed that the stress applied to the bulk materialincreases the magnetic anisotropy in the direction of application.However, the amplitude of this phenomenon depends closely on thecrystallographic orientation of the magnetic crystals before treatment:forging, extrusion, etc . . . .

In the case of any orientation whatsoever of the magnetic crystalsbefore treatement, slightly anisoitropic magnets, of direction ofdifficult magnetization parallel to the axis of extrusion, and isotropicin the other two directions, are obtained.

Furthermore, the direction of growth of the crystals is perpendicular tothe direction of easy magnetization.

It is therefore necessary to control the direction of growth of themagnetic crystals during the phase of solidification of the massivealloy. In fact, a unidirectional growth of crystals makes it possible todistribute the directions of easy magnetization in a plane perpendicularto the direction of growth, but not in a definite direction.

In this way, but judiciously selecting the direction of the stressduring welding with respect to the orientation of the crystals, it isthen possible to obtain completely isotropic magnets.

EXAMPLE 5

Three kilos of bulk-state alloy NdFeB are cast into a laterally cooledingot mould. A basaltic crystallization perpendicular to the cold wallis thus obtained. The whole is then extruded in a metallic envelope byisostatic extrusion in the form of a flat rectangular bar of 25×7 mmsection.

The ingot is placed so that the plane containing the directions of wasymagnetization is perpendicular to the rectangular bar and parallel tothe axis of extrusion. Anisotropic magnets, oriented in the direction ofthe flat face, are thus obtained, having the following characteristics:

H_(Ci) : 1000 kA/m

Br: 1.0 Tesla

H_(CB) : 650 kA/m

BH_(max) : 200 kJ/m³

The process according to the invention presents numerous advantages overthe process set forth in the preamble, for example:

the possibility of obtaining permanent magnets from the conversion ofcheaper raw materials;

easy and rapid to carry out, not employing any sophisticated equipment;

the absence of quasi-absence of dangers for the environment,particularly risks of explosion or fire, since powders are not employed.

In summary, this process is characterized by a consequent reduction incosts and the elimination of the dangers in manufacturing the magnets ofthe Iron/Boron/Rare Earth type, which are more and more sought after.

Consequently, this process may find numerous applications in themanufacture of permanent magnets, more particularly for manufacturingelectric motors, general-purpose motors, electronic apparatus,loud-speakers.

We claim:
 1. A process for preparing a magnet that is permanent atambient temperatures, comprising the steps ofselecting a bulk-statealloy having magnetic crystals, said alloy containing a mixture of aferromagnetic transition element, boron, and at least one element chosenfrom the group consisting of the rare earth elements and yttrium;heating and maintaing the bulk-state alloy in a controlled atmosphere toa temperature within a range of 400 degrees C. to 1050 degrees C.wherein the bulk-state allow is a two-phase mixture, one phase beingsolid and the other liquid; mechanically welding the two-phasebulk-state alloy with a deformation ratio of at least ten, sufficient tofracture magnetic crystals of said solid phase into smaller particlesizes; permitting the bulk-state alloy to cool; and annealing ortempering the bulk-state alloy at a temperature between about 600degrees C. and 1000 degrees C.
 2. The process according to claim 1wherein the bulk state alloy is a ternary alloy containing iron boronand one or more rare earth elements and wherein a tetragonal magneticphase, of said ternary alloy is present during the entire process. 3.The process according to claim 2 wherein the one or more rare earthelement is selected from a group consisting of neodymium, praseodymium,and both neodymium and praseodymium.
 4. The process according to claim1, wherein the mechanical welding is effected by hammering, rolling,forging or extrusion in a tight envelop made of an iron-based alloy.